The invention relates to an inhibitor as an additive for aqueous acid solutions, which is particularly suitable for hydrofluoric acid solutions and for pickling solutions and for cleaning pipelines and boiler parts made of steel and alloy steels.
In cleaning boiler installations, scale, rust and deposits must be removed with acid solutions, various inorganic and organic acids being used to this end, for example hydrochloric acid, sulphuric acid, amidosulphonic acid, citric acid, tartaric acid, or mixtures thereof. In recent times, solutions of hydrofluoric acid have been used successfully for this purpose. The concentration generally used is between 1 and 5% HF, with temperatures of up to 90.degree. C. A pickling solution made on a hydrofluoric acid base dissolves iron oxides faster than other acids under the same conditions. The fluoride ions can easily be precipitated in the waste water as insoluble calcium fluoride, and thus eliminated. One major disadvantage of hydrofluoric acid, however, is that it attacks steel very heavily, especially high-alloy heat-resistant steels, the surfaces of which in installations must be cleaned without bringing about any changes in dimension (for example special valves). In addition to St 35,8 steels, the following materials are often involved: 15Mo3, Vl3CrMo44, 10CrMo9 10, X 2oCrMoV12 1, X 8 CrNiNb 1613, x35 CrMo 17, 13CrMo44, and others. Thus, if the base material is to be protected, appropriate inhibitors must be added to the acid solutions. Most so-called pickling inhibitors are organic substances, small quantities of which are added to the acid solution.
A series of very good pickling inhibitors has become available for pickling steel in the usual acids, for example sulphuric or hydrochloric acid. However, attempts to use these inhibitors in hydrofluoric acid also, especially in treating high-alloy steels, have failed to produce satisfactory results.
The main requirements for a good pickling inhibitor for use with hydrofluoric acid are as follows:
(A) IT MUST ACT UPON HIGH-ALLOY STEELS EVEN WHEN USED IN SMALL QUANTITIES. Most known inhibitors are useless for this purpose or must be used in large quantities;
(B) GOOD SOLUBILITY IN THE PICKLING SOLUTION -- MANY PICKLING INHIBITORS HAVE POOR SOLUBILITY AND CAN THEREFORE ACT ONLY IN LOW CONCENTRATIONS. They frequently float to the top in the solution, and settle onto the surfaces of the metal under treatment, from which they are hard to remove. This disadvantage may be observed in the usual thio-urea derivatives used as inhibitors, for example dibutyl- or ditoluyl-urea. Attempts are therefore often made to improve the solubility of pickling inhibitors by adding dissolving agents. However, these additives are not sufficiently effective and have other disadvantages, for example, they cause foaming;
(C) RESISTANCE TO HIGH TEMPERATURES -- KNOWN INHIBITORS HAVE THE DISADVANTAGE OF NOT BEING RESISTANT TO HYDROFLUORIC ACID SOLUTIONS AND OF RAPIDLY LOSING THEIR EFFECTIVENESS, ESPECIALLY AT HIGH TEMPERATURES;
(D) FREEDOM FROM FOAMING -- IN THE CASE OF KNOWN INHIBITORS, THIS REQUIREMENT IS MET ONLY IN EXCEPTIONAL CASES. However, known non-foaming inhibitors have other disadvantages, for example poor solubility. Foaming is not detrimental to the pickling process itself, but raises difficulties in the processing of waste water. Foaming may be reduced to some extent by adding foam suppressants, but most of these are detrimental to waste water and their biological degradability is poor.